On the production line, mobile telephone handset manufacturers often face a need to cancel out carrier leakage at the antenna port, i.e. the port that delivers radio frequency signals to the handset's antenna for transmission. “Carrier leakage” is sometimes called “origin offset” or even “DC offset”. Presently, most manufacturers use simple but inaccurate or more accurate but time consuming methods for carrier leakage cancellation. An ideal carrier leakage cancellation scheme would provide accurate calibration in as short a time as possible in order to optimise the throughput of a handset production line.
A GSM handset manufacturer who wishes to bring a handset design into production must ensure that the handset passes a large number of tests (as defined in the 3GPP standards) relating to its RF performance. One of these test verifies whether the carrier leakage of the handset is below a specified value.
The 3GPP standards stipulate that the carrier leakage at the antenna port of a handset must always be better than −30 dBc when transmitting an EDGE or GMSK modulated signal. Achieving this level of performance may not seem very difficult but there are many factors that contribute towards carrier leakage and lead towards failure of the carrier leakage test.
It is conventional to regard the signal processing within a mobile telephone handset as divided into RF signal processing and base band signal processing. Typically, the RF processing is carried out by an RF or radio chip and the base band processing is performed by a base band chip. Amongst other tasks, a base band chip will typically be configured to take information that is to be transmitted from the handset and convert that information into a base band signal in quadrature format comprising I and Q differential signals. The quadrature format base band signal is then supplied to the radio chip for modulation on to an RF carrier signal. The RF chip then delivers the modulated RF signal into an antenna port leading to the handset's antenna for transmission.
In a handset of the general type described in the proceeding paragraph, there are two main contributors to carrier leakage appearing in the signal delivered by the RF chip to the antenna port. First, there may be DC offsets in the I and Q differential signals delivered by the base band chip to the radio chip. These DC offsets are due to the design of the base band chip. Second, defects in the radio chip may cause the RF carrier signal on to which the base band signal is modulated to leak into the output of the modulation process. As both of these contributors are completely uncorrelated, they may cancel one another but, alternatively, they may reinforce one another.
In order to pass carrier leakage tests, different handset manufacturers apply different strategies depending on the performance of their base band and radio chips. For example:
Very tight manufacturing tolerance may be imposed on the base band and radio chips themselves. However, this can decrease the yield of the manufacturing process thereby leading to increased costs.
Base band chips can be provided with technology that enables them to cancel DC offsets appearing in the differential I and Q signals that are delivered to the radio chip. Where such technology is used, the need to calibrate a handset to meet a carrier leakage test usually depends on the performance of the radio chip.
If the amount of carrier leakage attributable to a radio chip is statistically not good enough, which is commonly the case, then it becomes necessary to perform a calibration process on the RF signal that is delivered from the radio chip to the antenna port in order to reduce the total carrier leakage for the handset.
Typically, a calibration process of this kind uses registers within the base band chip to compensate for the total carrier leakage (i.e. arising from both the base band chip and the radio chip) by deliberately introducing corrective DC offsets in the differential I and Q signals that are delivered by the base band chip to the radio chip. However, since every handset moving along a production line with require calibration, it is important to make the calibration time as short as possible in order to maximise the throughput of the production line. A conventional calibration scheme of this kind will now be described.
The DC offsets provided to the differential I and Q signals delivered by the base band chip in order to suppress carrier leakage in the output of the RF chip are determined by a pair of programmable registers in the base band chip. One of the registers controls the DC offset that appears in the differential signal that is the I component of the signal delivered to the radio chip, whilst the other register provides the same function for the differential signal that is the Q component of the quadrature signal that is delivered to the radio chip.
The values held in the registers are stepped across their entire ranges and, for each possible combination of the values of the two registers, the carrier leakage at the output of the RF chip is measured using a power meter and recorded in a computer memory. Provided that the ranges used by the registers are wide enough, then the captured data should display a minimum carrier leakage value indicating the best pair of values to use in the two registers. FIG. 1 plots mean carrier leakage against the values of the two registers, which are referred to as Q OFFSET and I OFFSET in FIG. 1. Clearly, the pair of register values that produces the lowest carrier leakage at the output of the RF chip lies at the bottom of the well that appears in the mean carrier leakage surface in FIG. 1.
The calibration process described in the two preceding paragraphs suffers in that it can be time consuming to complete. The problem is exacerbated since manufacturers need to provide broad ranges for the registers to cater for the extreme carrier leakage values that might be encountered in handsets on a production line, even though the worst-case carrier leakage is likely to occur only infrequently.